In a ferroelectric film used in a FeRAM, the above properties can be obtained by heating the ferroelectric film to several hundreds degrees Celsius to crystallize the ferroelectric film, usually with oxygen being present. However, in the related art, the ferroelectric film is a poly-crystalline film, hence, even if the overall crystalline orientation of the ferroelectric film is aligned to be along a specified direction, for example, a direction perpendicular to a substrate, the orientation of the ferroelectric film is random in the other directions, and due to this, crystal grain boundary defects are inevitable. Related to this problem, in the related art, a semiconductor device having crystalline oxide films only shows properties of the oxide films.
On the other hand, in the related art, it is very difficult to form, on a silicon single crystal substrate, an oxide film having so-called epitaxial alignment, that is, the orientation of a crystal is aligned not only in a direction perpendicular to the substrate, but also in a direction parallel to the substrate.
In order to epitaxially grow a thin oxide film on a silicon single crystal substrate, it is necessary to utilize the orientation of the silicon single crystal substrate. However, the silicon single crystal substrate has the same chemical properties as metals, and the surface of the silicon single crystal substrate is apt to be oxidized when being exposed to an oxygen atmosphere at a high temperature, hence producing a silicon oxide film (SiOx). Because the silicon oxide film is not a crystal, and does not have a specific orientation, an oxide film cannot be epitaxially grown on the silicon oxide film.
In order for epitaxial growth, it is also important to minimize reactions and diffusions between the thin film to be grown and the silicon single crystal substrate. For this reason, not all of oxides can be grown by epitaxial growth on the silicon single crystal substrate. So far, only the following materials were reported to be able to be epitaxially grown on the silicon single crystal substrate, specifically, oxides of rare earth elements, such as, yttrium stabilized zirconia (YSZ) (J. Appl. Phys. Vol. 67, (1989) pp. 2447), magnesia-spinel (MgAl2O4: ISSCC Digest of Tech. Papers (1981) pp. 210), cerium dioxide (CeO2: Appl. Phys. Lett. Vol. 56 (1990) pp. 1332), and strontium titanate (SrTiO3) (Jpan. J. Appl. Phys. 30 (1990) L1415).
An index indicating crystalline quality of the thin oxide film epitaxially grown on the silicon single crystal substrate is a FWHM value (Full Width at Half Maximum) of the peak obtained in X-ray diffraction. The FWHM is deduced from a rocking curve obtained by scanning a θ axis with a 2θ axis of the X-ray diffraction peak being fixed, and equals the width of the rocking curve at half peak strength of the rocking curve. The FWHM expresses the degree of the crystal tilt in the thin film, and a smaller FWHM indicates that a state closer to the single crystalline state, that is, having a higher degree of crystalline orientation. Because when crystalline orientations of the thin film are aligned in a higher degree, the electric properties of the thin film become better, for example, the hysteresis property, or the leakage property are improved, it is important to grow a thin film having a FWHM as small as possible when the thin film is used in a device.
Materials having a perovskite structure, a typical example of which is barium titanate (BaTiO3), are ferroelectric materials, and are attractive because they possess piezoelectric properties, dielectric properties, pyroelectricity, semiconductivity, electric superconductivity. But in the related art, it is difficult to directly epitaxially grow the material having a perovskite structure on the silicon single crystal substrate. This can be attributed to, for example, production of an amorphous silicon oxide film (SiOx) on the silicon single crystal substrate, or formation of a reaction phase of silicide or others.
So far, an epitaxial perovskite film can be grown on the silicon single crystal substrate only from strontium titanate (SrTiO3). When epitaxially growing the perovskite film on the silicon single crystal substrate from strontium titanate (SrTiO3), a metallic strontium film is used in between as an intermediate layer. Because titanium (Ti) is liable to react with silicon (Si), in order to prevent the reactions between titanium and silicon, after the metallic strontium film is formed on the silicon substrate surface, strontium (Sr) and titanium (Ti) are supplied while oxygen gas is being flowed, thereby forming a strontium titanate film. If the intermediate metallic Sr layer is thin, Ti diffuses into the metallic Sr layer in the growing strontium titanate film, as if a structure is obtained by directly and epitaxially growing the strontium titanate film on the silicon single crystal substrate.
As described above, in order to epitaxially grow the strontium titanate (SrTiO3) film, it is essential to control the process at the level of atomic layer, and a technique called molecular beam epitaxy (MBE) is used for this purpose. As disclosed in Japanese Laid Open Patent Application No. 10-107216, a method is attempted to perform high vacuum laser ablation with a SrO target at a high vacuum of 10−8 Torr, form a strontium oxide (SrO) film as an intermediate layer temporarily, and then form a strontium titanate (SrTiO3) film. Even in this case, if the intermediate SrO layer is thin, Ti diffuses into the SrO layer, as if a structure is obtained by directly and epitaxially growing the strontium titanate film on the silicon single crystal substrate.
In addition, it is attempted to form an intermediate layer in order to prevent reactions between the silicon single crystal substrate and an oxide having the perovskite structure, and prevent formation of the SiOx phase. So far, the following materials were reported to be used as the intermediate layer, specifically, yttria partially stabilized zirconia (YSZ) (J. Appl. Phys. Vol. 67, (1989) pp. 2447), magnesia-spinel (MgAl2O4: ISSCC Digest of Tech. Papers (1981) pp. 210), and so on. When using the intermediate layer formed from these materials, the finally obtained film has a double-layer structure produced by stacking the intermediate layer and the perovskite film in order.
An yttria partially stabilized zirconia (YSZ) thin film epitaxially grown on the silicon single crystal substrate can be obtained by pulsed-laser deposition with an YSZ ceramic target. When growing the perovskite film on the yttria partially stabilized zirconia (YSZ) thin film formed on the silicon single crystal substrate, as reported in Appl. Phys. Lett. Vol. 67 (1995) pp. 1387), epitaxy occurs involving alignment of the (011) plane of the perovskite film relative to the (001) plane of the YSZ. However, because the direction of the spontaneous polarization is along the (001) direction in a perovskite film belonging to a tetragonal phase, if the (011) plane of the perovskite film is aligned, the direction of the spontaneous polarization is inclined by 45° relative to the substrate surface. In this case, the apparent polarization in the direction perpendicular to the substrate surface decreases, and this has an unfavorable effect on applications of FeRAM or piezoelectric actuators.
In the related art, it is a well known technique to epitaxially grow thin films of oxides of rare earth elements, such as, cerium dioxide (CeO2) or yttrium oxide (Y2O3) on the silicon single crystal substrate by pulsed-laser deposition using targets formed from materials having the corresponding compositions. However, since the thus obtained thin films of oxides of rare earth elements are aligned in the (011) plane relative to the silicon single crystal substrate, it is difficult to epitaxially grow the perovskite film aligned in the (100) plane.
For example, Japanese Laid Open Patent Application No. 10-120494 discloses background art of the present invention.